Reduce FLUX int8 test peak memory with sequential offload

[jiqing-feng]

Summary

Update the slow FLUX bitsandbytes int8 tests to use sequential CPU offload instead of model CPU offload.

enable_model_cpu_offload() can move an entire sub-model onto the GPU at once. For black-forest-labs/FLUX.1-dev, this can OOM on <=24 GB cards even when the T5 encoder and transformer are loaded from the pre-quantized int8 test checkpoint. Sequential CPU offload keeps peak memory lower by materializing one layer at a time, which lets the int8 FLUX tests run in more constrained environments.

The LoRA-loading assertion tolerance is also relaxed from 1e-3 to 2e-3 to account for small backend-specific numerical differences observed in the slow int8 path.

Changes

• Switch SlowBnb8bitFluxTests setup from enable_model_cpu_offload() to enable_sequential_cpu_offload().
• Document why sequential offload is needed for the FLUX int8 slow tests.
• Relax the test_lora_loading cosine-distance tolerance to 2e-3.

Validation

Run the affected slow tests:

RUN_SLOW=1 python -m pytest \
  tests/quantization/bnb/test_mixed_int8.py::SlowBnb8bitFluxTests::test_quality \
  tests/quantization/bnb/test_mixed_int8.py::SlowBnb8bitFluxTests::test_lora_loading \
  -x -s

Observed result:

2 passed

[jiqing-feng]

require change: huggingface/accelerate#4044 merged.